Add support for custom metadata

The parquet file format already supports extensible key/value storage in the column metadata and file metadata sections of the file. It would be nice if there was a facility in place for computing custom values to be placed in these key/value stores.

See: https://github.com/Parquet/parquet-mr/pull/185

**Reporter**: [Wesley Graham Peck](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=wesleypeck)

_{**Note**: *This issue was originally created as [PARQUET-15](https://issues.apache.org/jira/browse/PARQUET-15). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}